The biological conversion of cellulosic biomass to ethanol represents a major source of future energy, yet the current cost of converting biomass to fermentable sugars must be further reduced to render the process economically feasible. Most current strategies of ethanol production utilize simultaneous saccharification and fermentation (SSF) or simultaneous saccharification and co-fermentation (SSCF). The process configuration termed consolidated bioprocessing (CBP) would alleviate the financial strain of producing saccharolytic enzyme cocktails by combining all the necessary steps for ethanol production as an action of one organism.
One attractive candidate for a CBP organism is the gram-negative fermentative bacterium, Zymomonas naobilis. Z. mobilis has been studied for its high ethanol production rate, yield, and tolerance to the toxicity of the final product. In addition, Z. mobilis has the ability to ferment sugars at low pH, and has a naturally high tolerance to many of the inhibitory compounds found in lignocellulosic-derived hydrolysates. Furthermore, the use of the Entner-Doudoroff pathway allows Z. mobilis to achieve the near-theoretical maximum ethanol yields during fermentation while achieving relatively low biomass formation. Accordingly, Z. mobilis has been successfully used in SSF and SSCF processes. Additionally, Z. mobilis has been successfully engineered to ferment the pentose sugars, xylose and arabinose.
A necessary prerequisite to establishing an organism such as Z. mobilis as a CBP organism is to achieve high levels of cellulolytic enzyme expression. However, there is not yet a strong consensus on how to achieve maximal heterologous protein expression in Z. mobilis. Multiple groups have attempted heterologous expression of numerous genes including cellulolytic enzymes in Z. mobilis with varying degrees of success. The ability to achieve high levels of heterologous cellulase expression in CBP organisms such as Z. mobilis on a consistent basis needs further investigation.
While achieving high-level expression of cellulases is an important hurdle to clear in the development of a CBP organism, these enzymes must additionally be translocated to the extracellular media in order to come in contact with the substrate. One way to achieve this translocation is by harnessing the host-cell's secretion apparatus. However, there is very little knowledge of the capacity of Z. mobilis to secrete recombinant proteins.
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